Acrylic acid and the commodity acrylate esters (methyl, ethyl, butyl, and 2-ethylhexyl) comprise one of the most versatile monomer series for controlling polymer performance characteristics. These monomers all have an alpha beta unsaturated carboxyl structure and find extensive applications in surface coatings, adhesives and plastics. Furthermore, the sodium salt of polyacrylic acid is widely used as the superabsorbent polymer found in baby diapers. World production capacity for crude acrylic acid is almost eight billion pounds per year.
The first synthesis of acrylic acid was reported in 1843. This was accomplished via air oxidation of acrolein. Acrylic acid, a mature commodity chemical, has been well reviewed in the literature and has been commercially available since 1927. It has been manufactured on a commercial scale using several different technologies and raw materials, including the following:                1) Ethylene cyanohydrin process. The original version of this process reacted ethylene chlorohydrin with sodium cyanide to make ethylene cyanohydrin. A later modification reacted ethylene oxide with hydrogen cyanide. In both cases the ethylene cyanohydrin was treated with 85% sulfuric acid to yield acrylic acid and the by-product ammonium hydrogen sulfate.        2) Acetylene (Reppe) Process. The original or stoichiometeric version of this process reacted acetylene with nickel carbonyl and hydrochloric acid yielding acrylic acid, hydrogen gas and nickel chloride. A later version of the process used a nickel bromide/copper (II) bromide catalyst and reacted acetylene with carbon monoxide and water yielding acrylic acid.        3) Beta-Propiolactone Process. Ketene was reacted with formaldehyde in the presence of an aluminum chloride catalyst to obtain beta-propiolactone which was then treated with aqueous sulfuric or phosphoric acid, giving acrylic acid.        4) Acrylonitrile hydrolysis. Acrylonitrile is readily hydrolyzed with 85% sulfuric acid yielding acrylic acid and ammonium hydrogen sulfate by-product.        
All of these technologies have been replaced in commercial practice by the currently employed high temperature, vapor phase, two stage air oxidation of propylene process. The development of highly active and very selective heterogeneous catalysts was the key to the technology. In the first stage propylene is oxidized with air to acrolein and then fed directly to the second stage where the acrolein is further oxidized with air to acrylic acid. The catalysts used in the two stages are mixed metal oxides that have been optimized for their respective chemistries. The first stage catalyst is composed of mainly molybdenum and bismuth oxides with several other metals. The second stage catalyst is also a complex mixed metal oxide catalyst where the oxides employed are primarily of molybdenum and vanadium. Several other components have been incorporated in the catalyst to optimize activity and selectivity. Acrylic acid yields of 80-90% from propylene have been realized for these commercial catalyst systems.
The feed gas to the acrylic acid reactors is typically 6-9 vol % propylene and 12-15 vol % oxygen (coming from air) with a make up of either recycle gas or low pressure steam. The steam (or recycle gas) is added as a diluent to avoid forming a flammable mixture of propylene and oxygen. Typically the mixture is kept on the fuel rich side of the flammable envelope. A stoichiometric excess of oxygen is normally fed to the reactors to prevent reduction of the catalyst. The oxygen to propylene molar ratio is generally held between 1.6 and 2.0, which means that the exit gases contain oxygen.
The original acrylic acid processes used water as the diluent, which meant that the reactor product yielded an approximately 35% aqueous acrylic acid solution upon quenching and separation of the noncondensible gases in the quench or absorber tower. This low concentration of acrylic acid in water had to be recovered via a solvent based extraction followed by several distillation steps to generate a technical grade acrylic acid. Technical grade acrylic acid is used to prepare the higher purity glacial acrylic acid or to prepare acrylates, i.e. esters of acrylic acid. When recycle gas technology was introduced, the aqueous acrylic acid obtained in the quench tower was concentrated to approximately 65% which allowed the use of solvent based azeotropic distillation to remove the water. The crude acrylic acid after water removal was then subjected to several distillation steps to yield a technical grade acrylic acid. An alternate technology for recovery of the 65% aqueous acrylic acid involves the introduction of a high boiling solvent in the quench tower to absorb the acrylic acid via a solvent swap. The base of the quench tower yields acrylic acid dissolved in this high boiling solvent instead of water. The acrylic acid is then subjected to further distillation steps for recovery from the high boiling solvent to yield technical grade acrylic acid.
The problem associated with all these recovery systems is the high capital and operation costs associated with the purification towers. Each tower requires the addition of fresh inhibitor at the top of the tower to prevent polymer fouling. The inhibitor is very expensive and adds to the production cost of acrylic acid. Furthermore, these systems all require use of a solvent which adds cost and environmental concerns. It would be desirable to have an acrylic acid recovery system with reduced capital and operation costs.